In a GC/MS, a sample is separated into a series of components of a column in the GC section, and the components are introduced through the interface section into the MS section, where the components are sequentially mass analyzed. As shown in FIG. 3, carrier gas flows at a constant rate through the gas chromatographic column 12 which is heated by the column oven 13 at an appropriate temperature in the GC section 10. When a sample is injected into the injector 11, the sample is vaporized and carried by the carrier gas to the column 12. While the sample passes through the column 12, the sample is separated into its constituent components with respect to the passing time, whereby the components are sequentially introduced through the interface section 20 into the ionizing chamber 31 of the MS section 30. The molecules of the components are ionized by electrons (in the case of electron impact ionization) in the ionizing chamber 31, and the ions are converged by the ion lens 32 and introduced into the quadrupole filter 33 (or other mass spectrometer). On the four rods of the quadrupole filter 33 is applied a combined voltage of a DC (direct current) voltage and an RF (radio frequency) voltage, and such ions having a certain mass number (i.e., the ratio of mass to electrical charge m/z of the ion) corresponding to the applied voltage can pass the quadrupole filter 33 and enter the ion detector 34.
The column 12 is set at a temperature in the range of 100-300.degree. C., where the temperature depends on the component to be analyzed or on other factors. In many cases, the temperature is set according to the boiling point of the component. If the temperature of the sample decreases at the end of the column 12, the flow of the sample decreases and the accuracy of the analysis deteriorates. That is why the interface section 20 is heated at about the same temperature as that of the column 12. The ionizing chamber 31 is heated at a temperature appropriate for the stable ionization of the molecules, which is normally tens of degree centigrade lower than that of the interface section 20.
The interface section 20 and the ionizing chamber 31 are thus heated separately, in conventional GC/MSs, and a heating unit is respectively provided and attached to each of the interface section 20 and the ionizing chamber 31. A primary disadvantage of the conventional configuration is that it raises the cost of the GC/MS. Another disadvantage is that the maintenance of the ionizing chamber 31 (e.g., cleaning) is troublesome since the ionizing chamber 31 is contained in a vacuum housing 35 made of stainless steel. First the lead wires are detached from the housing 35; the ionizing chamber 31 is drawn out of the housing 35; and then the heating unit is detached from the ionizing chamber 31.
FIG. 4 shows an improved interface section 20, in which a capillary tube 21 connected to the gas chromatographic column (not shown in FIG. 4) is inserted into and held by the interface line 22 made of stainless steel, and a heating unit 23 is attached to the interface line 22. At the end of the interface line 22 toward the MS section 30 is attached a connecting ring 24 having an appropriate thermal resistance. The connecting ring 24 thermally connects the interface line 22 and the ionizing chamber 31. Since, as described before, the temperature of the ionizing chamber 31 can be lower than that of the interface line 22, the interface line 22 is directly heated by the heating unit 23, and the ionizing chamber 31 is heated by the heat from the interface line 22 conducted through the connecting ring 24. This configuration requires only a single heating unit 23 and no heating unit is needed in the ionizing chamber 31, which reduces the cost and facilitates the maintenance of the ionizing chamber 31.
The GC/MS shown in FIG. 4 still has some drawbacks. Since the ionizing chamber 31 is heated by the heat from the interface line 22, the temperature of the ionizing chamber 31 is not stabilized until the temperature of the interface line 22 is stabilized. This requires a prolonged period of time before a proper sample analysis can be started. Another drawback is that the long interface line 22 inevitably invites non-uniformity in the temperature since a part of the interface line 22 is directly heated by the heating unit 23 while the other part is not heated but is deprived of the heat by the connecting ring 24. This deteriorates the accuracy of the analysis. Still another drawback is that precise temperature control of the ionizing chamber 31 is difficult because it is heated indirectly via the connecting ring 24.